Monarch Butterflies Self-Medicate

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Monarch butterflies use medicinal plants to treat their offspring
for disease, before they even hatch, a new study finds.

Monarch
caterpillars feed on any of dozens of species of milkweed
plants, including some species that contain high levels of a
group of chemicals callled cardenolides. These chemicals do not
harm the caterpillars, but make them toxic to predators even
after they emerge as adults from their chrysalises.

As caterpillars, the monarchs are susceptible to gut invasions by
parasites, which persist when the caterpillars become adults. An
infected female passes on the
parasites when she lays her eggs.

"Several years ago we did experiments in which we reared monarch
caterpillars on two different species of milkweed, and found that
tropical milkweed reduced parasite infection, parasite growth and
the disease suffered by the monarchs, " said Jaap de Roode, an
evolutionary biologist at Emory University in Atlanta. "I then
wondered if monarchs could take advantage of this, by
preferentially using the tropical milkweed if they were
infected."

De Roode and his fellow researchers created an experiment in
which they raised monarchs and bred them in the lab. When new
butterflies were born, some were infected with the parasites.

Then, they mated uninfected females with infected males, placing
the females in a cage to lay their eggs. "The cage had both swamp
milkweed and tropical milkweed, which is much more toxic than
swamp. After the female laid their eggs, we counted them,"
De Roode said. "The infected females laid more of their eggs on
the tropical milkweed, while the uninfected females showed no
preference, which suggests that infected females were medicating
their offspring."

As for the next step in the research, De Roode plans to work with
a wider variety of milkweed and butterflies.

"We are studying
monarchs from different areas of the world, where they
encounter different species of milkweed, to investigate whether
this medication is a general phenomenon," said De Roode. "For
example, if there are other species in different parts of the
world, can the monarchs distinguish between these as well and
preferentially lay their eggs on medicinal species when they are
infected?"

De Roode added, "You look at these creatures that we think are
very simple, and they can do this fantastic thing. They look at
nature as a medicine cabinet. Other organisms learn to do this
from their parents, but monarchs don't learn self medication,
they do it innately."